In a servo control, the accuracy of the response of the system to an evolution of the setpoint is usually good. Specifically, the control tends to cancel out the difference between the setpoint and the value of the characteristic variable of the system monitored by the control sensor.
Nevertheless, the control subsystem may sustain an offset, tending to define a non-zero error even if the response of the system is perfectly suited to the setpoint. The offset may be due to the accuracy of the components of the control subsystem. The offset may evolve over time as a function of distinct parameters of the setpoint influencing the response of the system, parameters such as for example the evolution of the ambient temperature or else the wear of the components of the control subsystem.
The problem is currently solved by acting on each of the offsets of the various components and by trying to minimize their value by optimizing the design. Although costly because of the optimization necessary, this solution may be satisfactory at a given time, but does not prevent the evolution of the offset over time.
The invention can be applied in a backlighting command of liquid crystal screens used on aircraft instrument panels where it is necessary for the pilot of the aircraft to be able to see these screens irrespective of the ambient light in the cockpit.
In addition to the electronic offsets specific to any measurement subsystem, the preponderant offset for this backlighting application is generated by the ambient lighting in the cockpit, notably when the sun lights up the liquid crystal screen. A sufficient fraction of ambient light is then measured by the internal lighting sensor and skews the measurement of the latter. Since this interference lighting is added to that generated from the backlighting light source, the accuracy of the brightness seen by the pilot is degraded.